docs: def inject(self, request, response):
          """
          Inject the debug toolbar iframe into an HTML response.
          """
          # called in host app
          if not isinstance(response, Response):
              return
          settings = request.app[APP_KEY]['settings']
          response_html = response.body
          route = request.app.router['debugtoolbar.request']
          toolbar_url = route.url_for(request_id=request['id'])
  
          button_style = settings['button_style']
  
          css_path = request.app.router[STATIC_ROUTE_NAME].url_for(
              filename='css/toolbar_button.css')
  
          toolbar_css = toolbar_css_template % {'css_path': css_path}
          toolbar_html = toolbar_html_template % {
              'button_style': button_style,
              'css_path': css_path,
              'toolbar_url': toolbar_url}
  
          toolbar_html = toolbar_html.encode(response.charset or 'utf-8')
          toolbar_css = toolbar_css.encode(response.charset or 'utf-8')
          response_html = replace_insensitive(
              response_html, b'</head>', toolbar_css + b'</head>')
          response.body = replace_insensitive(
              response_html, b'</body>',
              toolbar_html + b'</body>')